The present disclosure relates generally to the problem of the settling of pigments over time in pigmented inks. This phenomenon leads to pen clogging and poor color consistency in the printed ink.
By increasing the viscosity of the ink, the settling can be decreased. However, increasing the ink viscosity also decreases the rate of pen refill during the ink firing act. This results in a decreased speed of printing.
Pigment particles can be milled to a very small particle diameter, for example, less than 100 nm. This also tends to decrease settling. However, it also tends to worsen light fade and ozone fade when the pigments are inkjet printed on media. Furthermore, certain specialty pigments, such as pearlescent pigments, have to be large in order to show a particular optical effect. In the case of pearlescent pigments, each pigment normally has a flat plate shape comparable in thickness to the wavelength of visible light, ˜500 nm, and several micrometers in diameter. This size of the pearlescent pigments combined with their high density of ˜3 g/cm3 causes them to quickly settle in typical ink-jet solvents.
If the ink delivery system is designed to constantly mix and re-circulate the pigments in the ink, this somewhat alleviates the problem of settling. However, this is not usually a practical solution and would generally add extra cost to the system.